The present invention relates to a device for measuring an amount of engine suction air in which an amount of air is measured by a Karman vortex sensor.
A variety of measuring methods in which an amount of flowing fluid is measured on the basis of a frequency of occurence of Karman vortexes produced downstream of a vortex generator provided in a conduit have been known in such as Japanese Utility Model Laid-Open Nos. 41665/1979 and 160625/1982.
In No. 41665/1979, the fact that ultrasonic signal wave transmitted and received through a flow passage is phase-modulated by Karman vortex generated in fluid flowing through the passage is utilized and, in No. 160625/1982, the fact that a resistance value of a thermister arranged in a flow passage and heated by a constant electric current varies according to Karman vortex is utilized.
In either of these conventional techniques, an output signal obtained is an analog electric signal which varies correspondingly to Karman vortex. When such technique is used as a suction air sensor for an engine fuel injection control, such output signal is "binarized" (converted from an analog signal into a two level digital signal) into a series of Karman vortex pulses a frequency of which is a frequency of occurrence of Karman vortex.
The conversion of the analog signal into the series of pulses is usually performed by comparing in a voltage comparator, the analog signal with a predetermined reference voltage, as shown in, for example, Japanese Patent Application Laid-Open No. 70131/1983.
In such Karman vortex sensor as applied to an engine as a suction air sensor, there is a problem that, when an opening degree of a throttle valve of the engine is low and thus suction air passes through the valve at high speed, the so-called "wind noise" is produced which affects the Karman vortex sensor in a suction conduit, causing its output to be overlapped with high frequency noise.
When such output signal is binarized according to the teaching of No. 70131/1983, the high frequency noise is also pulsed, causing subsequent signal processing difficulties.